Fuel cells are power generating units which have little exhaust and high energy efficiency, imposing little burden on the environment. Therefore, fuel cells are again being focused on, as global environmental protection has come highly, into value in recent years. Fuel cells are power generating units which hold high expectations for the future as power generating units in dispersion type power generation facilities of a relatively small scale, and of moveable bodies such as automobiles and ships. In addition, fuel cells are being focused on as power sources for compact mobile apparatuses and portable apparatuses, and are expected to be mounted inside cellular phones and personal computers instead of secondary batteries such as nickel hydrogen batteries and lithium ion batteries.
As for polymer electrolyte fuel cells, direct type fuel cells where fuel, such as methanol, is directly supplied, are being focused on, in addition to conventional polymer electrolyte fuel cells (hereinafter in some cases referred to as PEFC's), Where a hydrogen gas is used as fuel. Though direct type fuel cells have power that is lower than that of conventional PEFC's, the fuel is liquid, and no reformer is used, and therefore, there are advantages such that the energy density becomes high and the time for use of a cellular apparatus at full charge is long.
In a conventional polymer electrolyte fuel cell, anode and cathode electrodes where reactions for power generation occur, and a polymer electrolyte membrane that becomes a proton conductor between the anode and the cathode form a membrane electrode assembly (MEA), and this MEA forms a cell that is placed between separators as a unit. Here, the electrodes are formed of an electrode base for accelerating gas diffusion and collecting (supplying) electricity (also referred to as gas diffusing electrode or electricity collector), and an electrode catalyst layer that becomes an actual field for electrochemical reaction. In the anode electrode of a PEFC, for example, fuel, such as a hydrogen gas, generates protons and electrons through reaction in the catalyst layer of the anode electrode, where electrons are conducted to the electrode base and protons are conducted to the polymer electrolyte. Therefore, the anode electrode is required to have excellent gas diffusing properties, electron conductivity and proton conductivity. Meanwhile, in the cathode electrode, an oxidizing gas, such as oxygen or air, reacts, in the catalyst layer of the cathode electrode, with protons that have been conducted from the polymer electrolyte and electrons that have been conducted from the electrode base, so as to generate water. Therefore, in the cathode electrode, it also becomes necessary to efficiently discharge water that has been generated, in addition to gaining gas diffusing properties, electron conductivity and proton conductivity.
In addition, a direct type fuel cell where methanol is used as fuel from among PEFC's requires performance that is different from that of conventional PEFC's where a hydrogen gas is used as fuel. That is to say, in the anode electrode of a direct type fuel cell, fuel such as a methanol solution reacts in the catalyst layer of the anode electrode so as to generate protons, electrons and carbon dioxide, and conduct electrons to the electrode base, protons to the polymer electrolytes and release carbon dioxide to the outside of the system through the electrode base. Therefore, permeability of fuel such as a methanol solution and discharging properties of carbon dioxide are required, in addition to the properties that are required for the anode electrode of conventional PEFC's. Furthermore, in the cathode electrode of a direct type fuel cell, in addition to a reaction that is the same as that of conventional PEFC's, a reaction occurs, where fuel, such as methanol that has passed through the electrolyte membrane, and an oxidizing gas, such as oxygen or air, generate carbon dioxide and water in the catalyst layer of the cathode electrode. Therefore, the amount of water that is generated becomes greater than in conventional PEFC's, and it becomes necessary to discharge water more efficiently.
Conventional perfluorinated proton conductive polymers, such as Nafion (made by DuPont, trademark), have been utilized as polymer electrolytes. However, these perfluorinated proton conductive polymers have problems, such that formation of fuel, such as methanol, is great, and battery output and energy efficiency are not sufficient. In addition, the cost of perfluorinated proton conductive polymers is very high, because fluorine is used.
A variety of polymer electrolytes where an anionic group has been introduced into for example, a non-perfluorinated proton conductive polymer that is different from conventional perfluorinated proton conductive polymers have been proposed (US Unexamined, Patent Publication 2002/91225, U.S. Pat. No. 5,403,675, J. Membrane Sci., Vol. 197, 231-242 (2002)). When the introduced amount of an anion base is increased in order to gain high conductivity, however, these polymer electrolytes easily absorb water, and thus form a large water cluster in the polymer electrolyte, and the content of a low melting point water that is defined in the present specification is high, and therefore, the ratio of the amount of unfreezable water that is defined in the present specification is low, and there is a defect, such that fuel crossover, such as of methanol, is large. It is presumed that this is because fuel such as methanol easily penetrates through low melting point water.
In addition, a composition of a proton conductive polymer and another polymer has been proposed. A composite membrane made of sulfonated polyphenylene oxide and polyvinylidene fluoride (U.S. Pat. No. 6,103,414), for example, is known. In addition, a composite membrane made of sulfonated polystyrene and polyvinylidene fluoride (Published Japanese Translation of International Publication No. 2001-504636) is also known. The polymer electrolyte membranes which are described in these documents, however, are blend membranes of an ion conductive polymer and polyvinylidene fluoride, have poor compatibility and easily gain a large phase separated structure in the order of μm, where low melting point water or bulk water (defined in the present specification) exists between the phases, and therefore, the ratio of the amount of unfreezable water in the electrolyte is low, and it is difficult to achieve a high conductivity and suppression of fuel crossover at the same time. In addition, an assembly made of a proton conductive polymer and a copolymer of siloxane having a nitrogen atom containing group and a metal oxide (Unexamined Published Japanese Patent Application No. 2002-110200) is known. Though membranes made of an assembly of Nafion and siloxane (Polymers, Vol. 43, 2311-2320 (2002), J. Mater. Chem., Vol. 12, 834-837 (2002)) and the like are also known, the membranes described in these documents use Nafion, which is a perfluorinated proton conductive polymer, and therefore, it is difficult to achieve high proton conductivity and low fuel crossover at the same time, even when combined with another polymer.